The Growing Danger of Hurricane-Caused Floods

Hurricane Helene was one of the deadliest storms of 2024, with a death count of over 230 people. From the time it touched landfall in Florida on Thursday, September 26th to its dissipation on Monday, September 30th, it managed to impact areas of Florida, Georgia, South Carolina, North Carolina, Tennessee and Virginia. Part of the destruction came from high winds and storm surges it caused along the coast, but despite being a weather phenomenon that originated in the ocean, some of the worst devastation from the hurricane happened deeper inland – around the mountainous regions.

The Blue Ridge Mountains span all of the impacted states except for Florida. When the hurricane entered this region, the high elevation caused orographic uplift to occur: the transition from a lower altitude to a higher one cooled and condensed the air, leading to an increase in precipitation. More than 20 inches of rain fell in the region during the Hurricane’s duration. As another variable, rainfall over the preceding days meant that the ground was already saturated with water. Smaller mountain streams were unable to deal with the sudden spike in water flow and as a result, severe floods destroyed roads, bridges, and buildings. North Carolina bore the brunt of it, with at least 103 deaths in the State alone. Though the Hurricane did weaken once it moved inland, it only lessened to a post-tropical cyclone once it reached Tennessee.

Figure 1 Hurricane track data from https://www.nhc.noaa.gov/gis/archive_besttrack_results.php?id=al09&year=2024&name=Hurricane%20HELENE, streamflow data from https://waterdata.usgs.gov/nwis/rt

Rivers this far inland don’t often get hit by hurricane rainfall, but when they do, the impacts can be sudden and extreme, especially for smaller streams. For example, Figure 2 shows the surge in streamflow measured at two different stations in Georgia; one in the Coosa River (near Rome, Georgia), at an altitude of 553ft above sea level, and the other at 689ft in Holly Creek (near Ringgold, Georgia), which flows into the Conasauga River, a tributary of the Coosa River. That part of Holly Creek drains runoff from an area of only 64 square miles, whereas the Coosa River location collects runoff from 4040 square miles. The concentrated rainfall from Helene caused Holly Creek to spike to nearly 80 times its usual flow rate for that time of year, while the Coosa River saw a more gradual impact as the storm passed over its much larger basin.

Figure 2 Streamflow data from https://waterdata.usgs.gov/monitoring-location/02385800 and https://waterdata.usgs.gov/monitoring-location/02397000

These water gauge sites (shown in Figure 3) from across the Blue Ridge Mountains – all with over 1,000ft elevation – show noticeable spikes in water flow during the Hurricane’s duration compared to the days before or after; at their highest points the water flow is more than 10 times greater than the average (indicated by the red line). Individual gauge readings can be explored by clicking on a point on the map.

Figure 3 Streamflow data from https://waterdata.usgs.gov/nwis

Looking to the Future

A growing concern is that natural disasters such as hurricane-caused floods will become more prominent with the unusual temperatures caused by global warming. While specific circumstances – such as the hurricane path and rainfall in the preceding days – made Helene worse than expected, warmer air over the ocean will create an increase in moisture found in hurricanes. One study found that storms like Helene are up to 20 percent wetter over the southeastern coast of the US in the 21st century compared to conditions between 1979-2000. Due to this, the chances of hurricane-caused floods are likely to increase. In this case, strengthened infrastructure and evacuation plans are needed to combat the risk of flood-related deaths and destruction.

References

https://waterdata.usgs.gov/nwis

https://www.nhc.noaa.gov/gis/

https://sc.edu/uofsc/posts/2024/10/conversation-hurricane-helene-deadly-disaster-six-states.php

https://www.bbc.com/weather/articles/c1jdzwgepgzo

https://www.climameter.org/20240926-27-hurricane-helene